Cook pasta according to directions on box. When done, drain pasta water except leaving about 1/8 cup left in pot. Set aside.
Heat medium sized pan over medium heat and add olive oil, onion and mushrooms. Add black pepper and garlic powder and sauté for a couple of minutes or until translucent.
In the pot with pasta, add mushroom and onion mixture, butter and grated parmesan.
Stir together for another two minutes until pasta looks creamy and cheese has melted.